Numerous ILs cause the swelling of native biomass, which was seen as a good indicator of biomass solubility. Swelling occurs even at room temperature in poplar exposed to [EMIM][OAc]. The cross-sectional area of poplar cell walls expanded by 60 to 100% in 3 h in [EMIM][OAc]. After rinsing with deionized water, the wood structure contracted almost immediately [70, 71]. Significant swelling was also observed in Miscanthus switchgrass heated at 100°C in [EMIM][Cl] for 20 h [26]. The magnitude of wood swelling depended on the IL used in the pretreatment.

The swelling of pine (Pinus radiata) sapwood chips (dimensions 10 x 10 x 5 mm3) was studied in ILs consisting of the [BMIM] cation and several different anions. Little swelling was observed in 1-butyl-3-methylimida — zolium trifluoromethanesulfonate ([BMIM][CF3SO3]) even at 120°C. The ILs 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)2]) and [BMIM] [MeSO4] led to a swelling along the tangential direction (tangent to tree rings) of about 8%, which is larger than with just water (5%). The most dramatic swelling was observed in the tangential direction with [BMIM][Me2PO4] and [BMIM] [OAc] and the magnitude was temperature-dependent, from 15% at 90°C to 20% at 120°C [37]. Little expansion was observed in pine chips along the radial direction. [BMIM][N(CN)2] and [BMIM][MeSO4] caused the expansion along the axial direction, while 1-butyl-3-methylimidazolium dimethylphosphate ([BMIM] [Me2PO4]) and [BMIM][OAc] caused the reduction in the axial direction, most likely due to partial dissolution. The different swelling rates among ILs were attributed to the temperature-dependent viscosity [37].

However, swelling induced by the IL does not necessarily mean that the IL is a good solvent for cellulose and biomass. The swelling and dissolution of pine wood pulp fibers were studied in [BMIM][Cl], 1-allyl-3-methylimidazolium bromide ([AMIM][Br]), and butenylmethylimidazolium bromide. While the fibers swelled and dissolved in [BMIM][Cl], they swelled homogeneously in [AMIM][Br] and butenylmethylimidazolium bromide without dissolution. Both ILs penetrated the fibers quickly but did not disrupt the hydrogen bonding [90].